Space Prison by Tom Godwin

In SPACE PRISON (also published as THE SURVIVORS), a colony ship en route to planet Athena 500 light years away is attacked by the Gerns, an alien race with a deep hatred of humans. The colonists are enslaved and some are taken to work on Athena while others are taken to the prison planet Ragnarok.

Ragnarok, however, turns out to be an inhospitable world, populated by deadly, aggressive creatures. The novels follows the stranded humans through several generations as they try to survive there.